Characterising the Strength of Weathered Aggregates using Geophysical Methods for Engineering Purposes

Abstract

In New Zealand, with the expansion of infrastructure, such as housing developments, concrete and road works, the aggregate industry continues to grow. Natural aggregates are rock material that are used for construction purposes. With Auckland being New Zealand's largest urban centre, a large portion of our aggregate production supplies projects within the region. The main aggregate source for Auckland is greywacke. It is well noted that the strength of greywacke aggregates decreases with increased weathering. Currently, the main methods for testing aggregates in New Zealand are mechanical and chemical methods. These methods are used to quantify the strength and mineralogical compositions of the aggregates. It is important to test aggregates prior to use to understand how they will behave and avoid failures. This research aims to propose a new method for testing aggregate strength for the different weathering grades of greywacke. This was achieved using P- and S- waves combined with density and porosity to interpret the rock strength based on the rock's dynamic elastic properties. These measurements were given mechanical context through the use of the Slake Durability Index (Id2) as well as geochemical context through the use of XRD analysis. The study shows that P- and Swave speeds (and associated elastic moduli) decrease with increasing weathering grade. Porosity increased, while density decreased with weathering. Slake durability was less sensitive to weathering grades. The XRD data shows an increase in clay volume with weathering, indicating that the weathering is mostly chemical, and the transformation of greywacke forming minerals to clays increases porosity. We propose new empirical relations between these parameters and develop a weathering index for greywacke. We also test empirical relations from the literature used to model unconfined compressive strength. This research provides geophysical lab measurements as a new methodology for testing aggregates that we hope will be considered by industry professionals. The geophysical methods described here would provide a cost effective, fast and broadly applicable method for determining whether an aggregate sample is strong and t for purpose. In order for this methodology to be widely adopted, a shift in thinking to quantify strength in terms of dynamic elastic properties rather than only the traditional static elastic properties is required.